The present invention relates generally to method of sealing two or more layers of thermoplastic film and, more specifically, to a method of making thermal seals and perforations for thermoplastic films and bags.
For many years, thermoplastic bags have been widely used for a number of household and industrial purposes. Many bags have a rectangular structure comprising two layers of a c-folded thermoplastic film heat sealed along its sides with an open top. The c-folded thermoplastic web layers are integrally connected at the bottom. This c-folded structure has been adapted to form a variety of sizes and configurations that depend on the intended uses of bags. The bags are often formed from a web of film that is separated by bag widths via perforations. In recent years, bag manufacturers have developed new types of thermoplastic bags such as, for example, draw tape bags, handle bags and bags with protruding top edges that are more complicated.
The processes for making many existing thermoplastic bags involve expensive and complicated steps. These steps typically include forming the seals and later forming the perforations. One problem that may arise in forming the thermoplastic bags is the improper alignment of the perforations and seals. This problem is exacerbated by the increased cycle times that are being more often used in the manufacturing of thermoplastic bags. An extreme case is where the perforation wanders outside the two side seals resulting in one of the two adjacent bags not having a side seal. One method for reducing or eliminating this problem is the formation of a wider xe2x80x9cskirtxe2x80x9d or target area between the seals. This, however, is costly to the manufacturer because of the additional polymeric resin being used in forming the xe2x80x9cskirtxe2x80x9d along the sides that provides no benefit to the customer.
For the foregoing reasons, there exists a need for a process that overcomes such shortcomings discussed above.
According to one process, two thermoplastic layers are simultaneously sealed and a thermal perforation created between two thermoplastic layers. Two thermoplastic layers and a drum having at least one sealbar and perforation assembly are provided. The two thermoplastic layers are contacted to the sealbar and perforation assembly. The thermoplastic layers are sealed with two generally opposing parallel seals, while simultaneously creating a thermal perforation in the thermoplastic layers generally parallel to the opposing seals. The thermal perforation are located between the two opposing seals.
According to another process, a sealbar and perforating assembly is heated on a drum to form opposing seals and a thermal perforation between two thermoplastic layers. The sealbar and perforation assembly comprises a first sealing surface, a second sealing surface and a perforation surface. At least one heat source is provided to supply heat to the first and second sealing surfaces and a perforation surface. The material forming the first and second sealing surfaces has a lower coefficient of thermal conductivity than the material forming the perforation surface. The first and second sealing surfaces and the perforation surface use the at least one heat source. Heat is removed from the first and second sealing surfaces and the perforating surface. The first and second sealing surfaces are at a lower temperature than the temperature of the perforation surface.
According to one embodiment, a rotary drum simultaneously seals and creates a thermal perforation between two thermoplastic layers. The drum comprises at least one sealbar and perforation assembly. The assembly has a body, a perforation insert and at least one heating source. The body has a cavity therein and opposing first and second sealing surfaces. The perforation insert is located in the cavity and has a perforating surface. The at least one heating source is adapted to supply heat to the first and second sealing surfaces and the perforating surface.